140. _Main-Spines and Accessory Spines._--As accessory spines (Paracanthæ) we oppose to the main-spines (Protacanthæ), just described, all those processes which have no determining influence upon the formation of the skeleton as a whole, but are to be regarded as secondary constituents of the skeleton, or appendicular organs of inferior significance. They are developed in the utmost variety, sometimes as hairs or bristles, sometimes as thorns or clubs, either straight or curved (often zigzag), smooth or barbed; sometimes standing vertically upon the shell, or directed towards the centre, sometimes obliquely, or rising at a definite angle. In those SPUMELLARIA whose lattice-shell consists of several concentric spheres, the accessory spines generally arise from the outermost, the main-spines, on the contrary, from the innermost. In the NASSELLARIA, multifarious forms of accessory spines are especially developed in the order #Plectellaria#. In the PHÆODARIA they are often furnished with delicate appendages, _e.g._, anchor-hooks, spathillæ, coronets, &c. Among the ACANTHARIA the accessory spines which arise from the surface of the shell in the #Acanthophracta# are very characteristic. They are not radially disposed (like the similar superficial spines of the SPUMELLARIA), but parallel to the radial main-spines from whose transverse processes they arise. Since in all these #Acanthophracta# the twenty radial main-spines are opposite to each other in pairs, all the accessory spines (often several hundred) are parallel to ten different regularly disposed axes of the lattice-shell (Pls. 134-138). The skeletons of the Radiolaria, in addition to the general relations which have been discussed above, present numerous and important special differences in the various larger and smaller groups. These are indicated in detail in the descriptions of the legions, orders, and families in the systematic portion of this Report. {xciii}BIOGENETICAL SECTION. A SKETCH OF OUR KNOWLEDGE OF THE DEVELOPMENT OF THE RADIOLARIA IN THE YEAR 1884. ---- CHAPTER V.--ONTOGENY OR INDIVIDUAL DEVELOPMENT. (§§ 141-152.)